Recent years have observed researches for fixation and isolation of carbon dioxide being under intense study, as well as researches for use of carbon dioxide as a chemical raw material. In any case of research, carbon dioxide had better be collected by separation and concentration from the atmosphere, or from waste or exhaust gases discharged from stationary emission sources typified by a thermal power plant or the like.
For separation and concentration of carbon dioxide, old techniques of membrane separation, adsorption separation, and absorption separation have been based on to forward research and development.
The membrane separation method features a simple facility and operation, a clean process, and a small environmental load. The membrane separation method theoretically gives a possibility of reduced energy for separation, but for separation of carbon dioxide, it has not yet found a separation membrane meeting specifications to be satisfactory, such as of separation coefficient, transmission rate, and working temperature. Like this, in the field of membrane separation, current state is a level of basic studies being made of the separation membrane itself.
The adsorption separation method is implemented as a clean process of dry type that features reduced separation energy for high concentration gases to be the object. The adsorption separation method desorbs gas from adsorbant, making use of a pressure difference in a PSA (Pressure Swing Adsorption) method, a temperature difference in a TSA (Thermal Swing Adsorption) method, or both of them in a PTSA method, while the PSA method is typical for possible contraction of the time cycle. For collection of carbon dioxide from exhaust gases containing a high concentration of carbon dioxide like a waste gas of steel, it is implemented as a dry ice manufacture on a commercial base. However, in application to exhaust gases containing at most about 13% of carbon dioxide like a thermal power plant, too large separation energy constitutes a difficulty.
The absorption separation method is a method using an absorbant that chemically absorbs carbon dioxide, unlike the adsorption. The absorption separation method features a great amount of carbon dioxide to be collected per unit weight of absorbant. It however needs a great deal of energy to have the absorbant release carbon dioxide absorbed therein, as a problem. Even in the field of absorption separation, current state is a level of basic studies being made of new absorbants.
Like this, carbon dioxide separation and concentration techniques based on conventional techniques for separation and concentration, being membrane separation, adsorption separation, and absorption separation have not yet grown to a technical level applicable to separation and concentration of carbon dioxide of low concentration.
On the other hand, carbon dioxide separation and concentration methods by electrochemical measures have grown to be studied as new separation and concentration techniques.
For example, there is a known technology using a molten carbonate type fuel cell for electrochemical concentration of carbon dioxide from exhaust gases (refer to KASAI, H.,: “CO2 electrochemical separation by molten carbonate technology”, Prepr Pap Am Chem Soc Div Fuel Chem., Vol. 47, No. 1, pp. 69-70 (2002), and TAKENOBU Hidenori: “Study on a collection system of CO2 from coal fired thermal power plant exhaust gases using a molten carbonate type fuel cell” Chuugoku Denryoku Kabushiki Kaisha Giken Zihou, No. 98, pp. 55-65 (2002)). This technology has a better energy efficiency expectable in comparison with the above-noted PSA method, but is identical thereto in that energy is consumed. Further, not simply carbon dioxide but also oxygen is concurrently separated and concentrated, so that the concentrated gas results in a mixed gas of oxygen and carbon dioxide, requiring a separation of carbon dioxide from the concentrated mixed gas to implement a fixing process of carbon dioxide typified by a reduction of carbon dioxide or the like, thus needing additional energy for the separation. In addition, carbon dioxide from molten carbonate has a high vapor pressure, which disables separation and concentration of carbon dioxide from gases low of carbon dioxide concentration, as another problem.
Under such a condition of research and development for separation and concentration of carbon dioxide, an EMC (Electrochemically Modulated Complexation) has been proposed as a quite new electrochemical separation and concentration method (refer to Scovazzo, P., Koval, C,. Noble, R., “Electrochemical Separation and Concentration of <1% Carbon Dioxide from Nitrogen,” J. Electrochem. Soc., vol. 150, no. 5, pp. D91-D98, 2003).
Referring to FIG. 15 that schematically shows an electrochemical cell 200 including a container 203 in which an ionic liquid 202 is put and which is divided by a partitioning membrane 204 to form a CO2 capturing chamber 207 and a CO2 releasing chamber 208 and has electrodes 205 and 206 disposed in the CO2 capturing chamber 207 and the CO2 releasing chamber 208, respectively, the electrochemical separation and concentration method for carbon dioxide first introduces exhaust gases 211 containing carbon dioxide into ionic liquid 202 in the CO2 capturing chamber 207. The CO2 capturing chamber 207 has an emission port 209 formed in an upper wall portion thereof, and the CO2 releasing chamber 208 has a CO2 take-out port 210 formed in an upper wall portion thereof.
Then, it connects an external power supply 220 to the electrode 205 in CO2 capturing chamber 207 and the electrode 206 in CO2 releasing chamber 208, imposing a voltage for oxidation and reduction of a reductant Bred and an oxidant Box of a redox mediator B in ionic liquid 201 and 202. The reductant Bred of redox mediator B is bonded, as in formula (1) below, to CO2 in CO2 capturing chamber 207, and transfers the CO2 to the electrode 206 in CO2 releasing chamber 208. Then, the reductant Bred of redox mediator B bonded to CO2 is oxidized, as in formula (2) below, thus releasing CO2. Concurrently, redox mediators B not bonded to CO2 also are oxidized, as in formula (3) below, becoming Box.CO2+Bred→CO2Bred  (1)CO2Bred→CO2Box+e  (2)Bred→Box+e  (3)
The redox mediator B oxidized as in formula (2) releases CO2. As in formula (4) below, released CO2is taken out as CO2 separated from exhaust gas and concentrated.CO2Box→CO2+Box  (4)
The oxidant Box of redox mediator B permeates through the partitioning membrane 204, returning to the CO2 capturing chamber 207, where it is again reduced as in formula (5) below, to be provided for the capture of CO2.Box+e→Bred  (5)
This method implements electrochemically pumping CO2 to the CO2 releasing chamber 208, achieving separation and concentration. This method allows the separation and concentration of carbon dioxide in a pure state, enabling a separation and concentration also from gases containing a low concentration of carbon dioxide, which is advantageous over separation and concentration methods in the past as a matter course, and even to the separation and concentration of carbon dioxide using a molten carbonate type fuel cell.
It is noted that the wet-type solar cell (dye-sensitized solar cell) is a known technique, so-called Graetzel cell being known (refer to Japanese Patent Publication No. 2664194 (page 1, FIG. 1)).